Thermoplastic polyurethane elastomers are well-known. They are essentially the 1:1 reaction products of polymeric diols, optionally, one or more monomeric diols and aromatic diisocyanates. The polymers generally exhibit outstanding physical properties; however, in blow-molding applications they lack sufficient melt strength at normal blow molding processing temperatures. For instance, when one attempts to process thermoplastic polyurethanes by blow molding, the hollow segment of extrudate which is to be blown (also known as a parison), instead of hanging from the nozzle, often drops off before blowing can be effected. When attempts are made to blow film, the polymer tends to fold back on the surface of the extrusion die.
The properties and, therefore, the utility of physical blends of polymers are strongly dependent on the degree of compatibility of the components. A very small number of amorphous polymer pairs are thermodynamically compatible, i.e., truly soluble in each other. Blends which display an intermediate degree of compatibility, i.e., between incompatible and completely or thermodynamically compatible, also are known to exist. These systems are called "mechanically" compatible or semi-compatible. Examples include a blend of a butadiene-acrylonitrile copolymer and a styrene-acrylonitrile copolymer (known as ABS plastic) and a blend of polystyrene and SBR rubber (known as impact polystyrene.) These blends are essentially identical to the incompatible blends in their thermal behavior, i.e., they display two major glass transition temperatures (Tg). However, their morphology is finer and they are more translucent. They display a higher degree of interphase adhesion, which is reflected in improved mechanical properties, e.g., melt strength, see Noshay and McGrath, Block Copolymers: Overview and Critical Survey, Academic Press, N.Y. 1977, pages 1-10.